For years people have insisted that we have good vapour barriers to protect wall and attics from condensation. But others have railed against vapour barriers, claiming they cause condensation problems inside the house.
Now the facts are known -- both groups are wrong and most people confuse vapour barriers with air barriers (search keyword "air barriers" for the title "AIR BARRIERS").
Water vapour manages to filter its way through most building materials; we call this diffusion. Although all building materials slow diffusion to a certain extent, only those that almost totally block it are officially classified as vapour barriers and deemed acceptable for new construction standards. Permeability is the technical measure of how easily vapour will get through a material and only materials rated as less than 1 perm are considered vapour barriers. Aluminum foil, polyethylene films, aluminum paints, latex vapour barrier paint, vinyl wallpaper, and several coats of oil paint all qualify in decreasing order of effectiveness.
The polyethylene films are the far the preferred materials for vapour barriers because they come in large sheets, have few joints where air can leak by, and can be efficiently sealed around openings such as windows, doors and electrical boxes. So they create an air barrier while also meeting the code requirements for a vapour barrier.